Dies are respective portions of semiconductor wafers that have been cut up into individual units after completion of wafer-fabrication steps. Each die has substantially all its specified circuit layers and is ready for packaging to make finished devices.
A die pick-up tool comprises a pick-up “head” mounted to and controllably moved by a robotic mechanism. The die pick-up head is the actual device that picks up the die for movement into position for execution of a die-packaging step that usually involves bonding the die to a substrate.
Die packaging is normally the final process in the long chain of processes for manufacturing semiconductor integrated circuits and the like. Die packaging is a multi-disciplinary technology that typically involves many steps. The technology is critically important because it has a direct impact on device performance and reliability. “Packaging” as used herein encompasses any of various techniques of preparing a die for actual use as an electronic device. In many instances, packaging includes mounting the die on a lead-frame or analogous item and encapsulating the die to seal and protect the die from the external environment, while allowing peripheral portions of the lead-frame to extend outside the package for making external electrical connections to and from the die. Packaging also can facilitate the conduction of heat away from the die during use. More recent types of packaging, currently being used increasingly widely, simply involve mounting the die on a substrate or carrier, with which the die directly makes the required external connections, without forming a discrete capsule around the individual die beforehand (and usually without using a lead-frame). Modern “flip-chip” packaging schemes are in this latter category.
Die packaging is prefaced by wafer dicing and/or scribing by which a wafer that has completed the various “wafer-fabrication” steps is cut up into individual dies. The combined processes of dicing and packaging are usually called semiconductor “assembly” processes. During dicing, the finished semiconductor wafer is cut using a dicing saw, laser beam, or the like so as to separate the dies from one another.
In substantially all packaging operations involving dies, each die is bonded to some kind of substrate (e.g., ceramic, metal, plastic, or other suitable material) to provide the die with mechanical support, ruggedness, and a heat-removal capability. Mounting dies to substrates involves sophisticated robotics that execute the required steps in a highly accurate and precise manner while ensuring an adequate level of process cleanliness, achieving a desired rate of processing (“throughput”), and avoiding damage to the delicate dies. To bond a die to a bond site on a substrate, the die typically is picked up by a die pick-up head (sometimes called a “collet”) that is robotically moved into position adjacent (usually vertically above) the bond site. A small amount of adhesive is placed between the die and bond site, and the pick-up head is moved to lower the die onto and adhere the die to the bond site.
A conventional die pick-up head comprises a die-contact face (usually facing downward) that is supported (usually from above) by a stem. The face usually is made of a compliant material to avoid damaging the surface of the die. At least one vacuum-orifice opens onto the face. The vacuum-orifice is connected to a conduit that usually extends upward inside the stem. The conduit is connected to a vacuum pump. Whenever the face is in close proximity to a die for pick-up, the conduit and orifice are evacuated by the pump to attract and hold the die to the face. The die remains held to the face so long as the vacuum in the conduit is maintained. Meanwhile, the head moves the die into position over a bond site on the substrate and lowers the die onto the bond site. At time of placement of the die at the bond site, evacuation of the conduit and orifice is halted to release the die from the face.
The adhesive is applied to the bond site before the die is lowered onto the bond site. The amount of adhesive and the manner of applying it are tightly controlled. A widely used method is providing the adhesive in the form of a flexible film (called “die attach”) that is robotically applied to the bond site. Bonding a die to the substrate using a die attach usually is prefaced by localized application of heat or other energy to the bond site to soften the die attach sufficiently for achieving a bond while avoiding excessive liquefaction of the adhesive. In other die-bonding processes adhesive (e.g., epoxy) is applied as a small dollop to the bond site just before lowering the die onto it.
In most conventional die pick-up heads, the face is planar. Examples of planar-head faces are discussed in U.S. Pat. No. 4,875,279 to Sakiadis; U.S. Pat. No. 6,742,561 to Nam et al., and U.S. Pat. No. 6,995,468 to Abe et al., all incorporated herein by reference.
After placement of the die on the bond site, and while the adhesive is sufficiently softened, the pick-up head usually applies a small amount of downward force to the die to cause the adhesive to spread out between the die and bond site and thus form a thin adhesive film between the die and bond site. A persistent problem with this approach, particularly when using heads having planar faces, is the frequent formation of “film voids” (empty regions in the adhesive film due to entrapped or drawn-in air) between the die and bond site. Film voids are not visible, and thus are not readily detectable, but they can substantially reduce heat conduction from the die to the substrate in a finished device. Poor heat conduction can result in increased probability of device failure. Film voids also can substantially reduce the ruggedness of the finished device.
Unfortunately, current approaches for solving the problem of film voids, such as using pick-up heads having curved faces, have shown some promise with respect to achieving good bonding. But, pick-up heads in general do more than simply bond the die; they must also be very effective in picking up dies and transporting them without mishap. Unfortunately, current pick-up heads having curved faces exhibit difficulty in consistently maintaining the desired vacuum for holding the die to the face. As a result, the pick-up heads exhibit less than optimal performance of holding and transferring dies, especially when used at high speed and/or when manipulating large dies.
In view of the foregoing, a need exists for die pick-up tools and heads that perform all three tasks (pick-up, transport, and bonding) with a very high degree of consistent and reliable performance at high-speed operation and with large dies.